Traditionally pest control has been dominated by the use of chemical insecticides. Although they are fast acting, these chemicals are sometimes environmentally unattractive. In addition, many chemicals used in insect pest control are not species-specific and may affect non-target vertebrates and invertebrates as well as the target pest. These chemicals or their by-products can sometimes persist in the environment for long periods of time.
The development and use of pest biology, population dynamics, silvicultural practice, natural control agents (i.e., parasitoids, pathogens), and improved operational forest pest management practices are new tools for forest management. Biological control, the use of living organisms to control insect pests, has become increasingly more acceptable as a means of successfully controlling pests. For example, the bio-insecticide Bacillus thuringiensis (Bt), is used for control of spruce budworm and gypsy moth larvae. However, some recent concerns over the specificity of Bt have resulted in the recommendation that it not be used in areas where there are endangered Lepidoptera. Ecological interests have resulted in a shift in emphasis to examine and develop other microbial products, including the insect viruses.
Insect viruses are naturally occurring insect pathogens that are considered to be host specific and environmentally safe. They can persist from year to year to impact on several generations of insects. There are over 1200 insect viruses (nucleopolyhedroviruses, granulovirus, entomopoxviruses, cypovirus and others) that have potential for insect control.
One problem associated with several natural insect viruses is that there is a time delay between the viral entry into the insect body and the lethal infection. Insect viruses must be ingested by larvae to allow infection. Occlusion bodies containing virus particles contaminating the foliage are eaten and dissolved by the insect's midgut juices, releasing virus particles. These particles pass through the gut cells and infect tracheal and other body tissues of the host larva. Over a typical period of 15 days, the virus replicates in susceptible tissues eventually causing death. Infected larvae still feed, during this time; however, and hence significant defoliation of plants still can occur in the time interval between ingestion of virus and insect death. This feeding damage is an inherent problem with using natural insect viruses.
Another problem associated with natural insect viruses is lack of virulence. For example, extensive field trials have shown that the spruce budworm nucleopolyhedrovirus (CfMNPV) will infect populations of spruce budworm, but has not caused epizootics that result in large scale mortality and population reduction (Cunningham and House, 1984, Choristoneura fumiferana (clemens), Spruce budworm (Lepidoptera: Tortricidae); B. Viruses: Application and Assessment. In Biological Control Programmes against Insect and Weeds in Canada 1969-1980, Kelleher, J. S. and Hulme, M. A., eds., Commonwealth Agricultural Bureau, Slough, England).
Many strategies have been adopted to decrease the feeding damage caused by infected insects. One strategy is the application of virus formulations containing "virus enhancers" to early instar larvae so that infection occurs faster, preventing serious defoliation. Unfortunately, this strategy cannot be used if the insect is evasive or if large amounts of the insect virus are unavailable. Such is the case for the control of the spruce budworm, Choristoneura fumiferana (Clem), with nucleopolyhedrovirus.
The development of biotechnology provides tools to genetically modify insect viruses to enhance their effectiveness. Genes encoding toxins (scorpion/mite toxin), enzymes (juvenile hormone (JH) esterase), neuropeptides (prothoracicotropic hormone), and eclosion hormone have been introduced into the viral genome by various research groups (Bonning et al., Annu. Rev. Entomol. 85: 437-446). These genes encode secretory proteins or peptides which assert their functions outside of virus infected cells. Inserting the JH esterase gene into the alfalfa looper nucleopolyhedrovirus (AcMNPV) results in the secretion of the enzyme JH esterase into the hemolymph and improves the virus as a control agent. Scorpion toxin and mite toxin have also been inserted into AcMNPV. These proteins are neurotoxins that are secreted into the hemolymph and act on the nervous system. A major drawback of these transgenic viruses is that the foreign genes encode secretory products which have to act outside of infected cells, e.g., in the hemolymph. These gene products run the risk of being degraded or eliminated by the insect's detoxifying system.
There is still a need to develop new transgenic viruses as biopesticides. Especially there is a need to construct transgenic viruses by introducing new types of foreign genes into the viral genome.